Gears can be categorized into various types according to usage application, and detailed profiles are set for teeth of gears to suit usage application and usage environment. For example, an involute tooth profile tracing an involute curve of a pressure angle θ (generally, θ=20°) in a tooth profile direction has advantages in terms of usability such as an advantage that gears adjacent to each other rotate smoothly even when the distance between the centers of the gears changes to a certain degree. Moreover, breakage and the like of a tooth due to partial contact of a tooth face can be prevented by providing crowning on the tooth face in which a center portion of the tooth in a tooth trace direction bulges and both end portions of the tooth have small thickness.
Detailed profiles like ones described above can be easily set for a gear by using a gear grinding device. However, in a case of combining detailed profiles like ones described above and setting the combined profile for one gear, for example, in a case where crowning is set for the involute tooth profile described above, it is difficult to uniformly machine the gear with both of the involute curve and the crowning. Providing the crowning causes the involute curve and the pressure angle θ to vary in the tooth trace direction and the tooth face is twisted.
Such twisting of the tooth face is called bias and the degree of twisting is called bias amount. Techniques of bias correction and bias adjustment for setting the bias amount to zero to eliminate the twisting of the tooth face are being popularly studied.